The invention relates to a process of preparing alkali metal titanates. In particular, the invention relates to a lithium titanate product that has a high Li:Ti molar ratio, and the use thereof.
Various lithium titanates and their properties have been studied extensively due to the potential use of the material in battery applications. Lithium titanate is used as an anode electrode material, since a high energy density is provided with it and it is rechargeable. Typically, lithium batteries are used in consumer and entertainment electronics, such as computers, mobile phones and cameras.
The lithium ions in the lithium titanate settle in a TiO2 crystal lattice, forming an LixTiO2 form in an octahedral anatase structure, at its easiest. The goal is to run as many lithium ions as possible into the structure, but their number is however limited by the repulsive, coulombic interactions between the lithium ions in the structure. It has been suggested that the maximum amount of Li in a TiO2 anatase crystal is x=0.5-1, depending on the temperature and the method of synthesis.
Lithium titanates have previously been prepared in various ways; particularly, through solid-state reactions that take place at high temperatures of 200-1000° C. Typically, the products thus provided are comprised of tetralithium titanate Li4Ti5O12. Additionally, various titanates that have a Li:Ti ratio, such as various LixTiyO4 structures, wherein the x:y varies within 0.5-2 and, and for example, the Li4TiO4 and Li2Ti3O7 structures, as well LixTiyO12, have been prepared, whereby x=3-5 and y=4-6, and Li2TiO3, for example. It is a known fact that when the ratio of lithium to titanium increases to two, such as in the Li2TiO3 structure, the usability of the material in battery or accumulator applications decreases. This is due to the fact that after the first discharge of the battery, oxide is generated, to which the Li ion can no longer be returned. Instead, it has been observed that an LiTiO2 type of structure, which has O3 levels that enable the movement of lithium, is functional, although its Li:Ti ratio remains low. Other possible lithium titanate structures are disclosed, for example, in the publications ICDD 1998, ISSN 1084-3116, Powder Diffraction File, Release 1998: Datasets 1-48 plus 70-85.
In the article Zhang, D. et al., J. Ind. Eng. Chem., vol. 13, No. 1, 2007 p. 92-96, nano-crystalline LiTiO2 has been synthesized by a hydrothermal process. In this process, LiOH.H2O was dissolved in ion-exchanged water and the solution was mixed with TiO2 powder (Degussa P25). The mixture was placed in an autoclave and heated to 180° C., and to a corresponding pressure for 24 hours, after which it was cooled in air, filtered and washed with water and acetone. In this way, cubic LiTiO2 was provided, its lattice constant thus being 4.14 Å, and its average crystal size was about 30 nm. The Li:Ti ratio of the product thus obtained is one at the maximum, whereby the Li content remains relatively low. The process cannot be used to flexibly adjust the crystal size of the generated product. Furthermore, the production conditions, such as the high temperature and pressure, are not the best possible regarding safety, when operating in an industrial environment.
The article Wagemaker, M. et al. J. Am. Chem. Soc. 2007, 129, 4323-4327 describes the effect of the particle size on the insertion of lithium into TiO2 single anatase crystals. In the study, TiO2 powder was dispersed in hexane, and n-butyl lithium was added to the mixture, while slowly stirring. The stirring was continued for 3 days, after which the mixture was filtered, washed with hexane and dried. All of the production stages were carried out in an argon cabinet. In this way, LixTiO2 was obtained, wherein x=0, 0.12, 0.4 or 0.8. Furthermore, it was observed that along with a decrease in the crystal size, it was possible to increase the amount of lithium. The largest amount of lithium was run into the TiO2 crystal structure, when the crystal size was the lowest possible, 7 nm, whereby the composition corresponded to the formula Li1TiO2, and the crystal structure had a tetragonal I4l/amd symmetry. In a product according to the description, the amount of Li still remains low, and the production conditions are not easy to implement industrially.
The Patent Specification EP1409409 discloses the production of tetralithium titanate, Li4Ti5O12, the particle size of which can be adjusted to within 5 and 2000 nm, and its BET is 1-400 m2/g. In the production method, a mixture is formed that contains a titanium compound and a lithium compound. This mixture is evaporated to form the mixture of titanium compound and lithium compound. The evaporation is carried out by spray-drying the mixture at a temperature that is higher than the boiling point of the medium of the mixture, preferably water, i.e., over 100° C., but still lower than the temperature, at which the reaction between the lithium and titanium compounds essentially takes place. The titanium compound can be titanium oxychloride or, according to the examples, titanium chloride, or an amorphous oxidized titanium compound, such as titanium dioxide. The lithium compound can be lithium chloride, lithium oxychloride, lithium nitrate, lithium hydroxide or a mixture thereof. The mixture provided by the evaporation is calcined to produce a reaction between the lithium and titanium and to form the lithium titanate product.
The abstract of the Patent Specification JP09309727 (WPINDEX AN: 1998-071742 [07]) describes the production of laminate-structured, elongated lithium titanate particles from titanic acid and a lithium compound from an ammonium compound-bearing aqueous solution at a temperature of 50° C. The presence of ammonia has a desired effect on the morphology of the compound that is precipitating, but ammonia easily causes process-technical problems, when evaporating readily as ammonia when the pH raises to above 7, and the nitrogen contained in the used solution forms an environmental problem in further processing.
The purpose of the present invention is to disclose a lithium titanate product that has a high lithium content.
Another purpose of the present invention is to provide a low-temperature process for the production of alkali metal titanates on an industrial scale, and for the production of the said lithium titanate product, in particular.